Single-crystal cathode materials for lithium-ion batteries have attracted increasing interest in providing greater capacity retention than their polycrystalline counterparts. However, after being cycled at high voltages, these single-crystal materials exhibit severe structural instability and capacity fade. Understanding how the surface structural changes determine the performance degradation over cycling is crucial, but remains elusive. Here, we investigate the correlation of the surface structure, internal strain, and capacity deterioration by using operando X-ray spectroscopy imaging and nano-tomography. We directly observe a close correlation between surface chemistry and phase distribution from homogeneity to heterogeneity, which induces heterogeneous internal strain within the particle and the resulting structural/performance degradation during cycling. We also discover that surface chemistry can significantly enhance the cyclic performance. Our modified process effectively regulates the performance fade issue of single-crystal cathode and provides new insights for improved design of high-capacity battery materials.

for the battery technologies. However, the main impediment for the practical application of lithium metal batteries is the lithium dendrite formation. [5] It not only penetrates the separator to induce the short circuit of the batteries, [6] but also generates high surface area in the anode to accelerate the unwanted side reactions between electrolytes and lithium metal, resulting in the electrolyte depletion and the subsequent battery failure. [7][8][9] Up to now, many strategies targeting lithium dendrites suppression rely on the "internal strategies," i.e., the modification or optimization of the components inside the cells. Those strategies include electrolyte optimization, artificial solid electrolyte interphase (SEI) design, and synthesis of 3D current collector. [1] Electrolyte additives such as LiF, [10] LiNO 3 , [11] and Li 2 S x [12] were chosen to form stable SEI on the surface of Li anode to suppress the dendrite growth. [13] For creating artificial SEI layer on the anode, gas treatment [14] (N 2 , O 2 , CO 2 , or SO 2 ), liquid treatment (Li 3 PO 4[15] and Cu 3 N/styrene−butadiene rubber [16] ), and physical deposition of nanofilm (Al 2 O 3 , [17] carbon, [18] and organic polymer [19] ) are the three typical methods by accessing the internal interface of the battery. The rational design of 3D current collectors also helps to inhibit dendritic growth. [20] One type of 3D current collectors is lithiophilic matrix such as lithiophilic-lithiophobic gradient interfacial layer, [21] N-doped graphene, [22] and metal−organic framework, [23] which redistributes Li-ions to the anode surface through chemical bonding interactions to achieve uniform lithium deposition. The other type is conductive matrix including porous carbon, [7] graphene matrix, [24] 3D-ordered macroporous Cu, [25] and fibrous metal felt [26] that reduces dendritic growth by reducing the current density with a large surface area. [27,28] Although these "internal strategies" could effectively suppress the dendrite formation, cell stability becomes a concern due to the change of the cell environment such as the use of additives and the modification of the electrode.Introducing an "external strategy," by using external magnetic field to rearrange the Li + concentration on the anode surface, could achieve a uniform lithium deposition. The latest study shows that the growth of Li dendrites stems from the nonuniform Li + concentration on the electrode surface. [29] Lithium metal is the most attractive anode material due to its extremely high specific capacity, minimum potential, and low density. However, uncontrollable growth of lithium dendrite results in severe safety and cycling stability concerns, which hinders the application in next generation secondary batteries. In this paper, a new and facile method imposing a magnetic field to lithium metal anodes is proposed. That is, the lithium ions suffering Lorentz force due to the electromagnetic fields are put into spiral motion causing magnetohydrodynamics (MHD) effect. This MHD effect can effecti...

Rosenfeld's perturbative method [J. Chem. Phys. 98, 8126 (1993)]] for constructing the Helmholtz energy functional of classical systems is applied to studying inhomogeneous Lennard-Jones fluids, in which the key input-the bulk direct correlation function-is obtained from the first-order mean-spherical approximation (FMSA) [J. Chem. Phys. 118, 4140 (2003)]]. Preserving its high fidelity at the bulk limit, the FMSA shows stable and satisfactory performance for a variety of inhomogeneous Lennard-Jones fluids including those near hard walls, inside slit pores, and around colloidal particles. In addition, the inhomogeneous FMSA reproduces reliably the radial distribution function at its bulk limit. The FMSA is found, in particular, much better than the mean-field theory for fluids near hard surfaces. Unlike alternative non-mean-field approaches, the FMSA is computationally as efficient as the mean-field theory, free of any numerical determination of structure information, weight functions, or empirical parameters.

Lithium (Li) dendrites formed from nonuniform Li deposition limit potential uses of Li metal as an anode material. Here, we report a lithiophilic 3D porous CuZn current collector that mitigates dendrite formation and realizes the high stability of Li anodes. As opposed to the common metal current collector of copper (Cu), calculations and in situ experiments demonstrate that copper zinc (CuZn) alloys found in commercially available brass are lithiophilic and promote uniform Li deposition. Facile dealloying methods are applied to provide sufficient Li deposition and volume expansion space in brass sheets. Residual CuZn alloys in the framework are found to induce uniform Li deposition and stabilize Li dendrite growth. The optimal current collector (2h-3D CuZn) runs smoothly over 220 cycles at 0.5 mA cm −2 with improved Coulombic efficiencies at high current density. This demonstrates an economic and facile CuZn preparation method that unlocks the full potential of Li anodes and provides new Li battery stabilization approaches.

The development of novel electrochemical energy storage devices is a grand challenge. Here, an aqueous ammonium‐ion hybrid supercapacitor (A‐HSC), consisting of a layered δ‐MnO2 based cathode, an activated carbon cloth anode, and an aqueous (NH4)2SO4 electrolyte is developed. The aqueous A‐HSC demonstrates an ultrahigh areal capacitance of 1550 mF cm−2 with a wide voltage window of 2.0 V. An amenable peak areal energy density (861.2 μWh cm−2) and a decent capacitance retention (72.2% after 5000 cycles) are also achieved, surpassing traditional metal‐ion hybrid supercapacitors. Ex situ characterizations reveal that NH4+ intercalation/deintercalation in the layered δ‐MnO2 is accompanied by hydrogen bond formation/breaking. This work proposes a new paradigm for electrochemical energy storage.